Susceptor designs for silicon carbide thin films

ABSTRACT

A susceptor is disclosed for minimizing or eliminating thermal gradients that affect a substrate wafer during epitaxial growth. The susceptor comprises a first susceptor portion including a surface for receiving a semiconductor substrate wafer thereon, and a second susceptor portion facing the substrate-receiving surface and spaced from the substrate-receiving surface. The spacing is sufficiently large to permit the flow of gases therebetween for epitaxial growth on a substrate on the surface, while small enough for the second susceptor portion to heat the exposed face of a substrate to substantially the same temperature as the first susceptor portion heats the face of a substrate that is in direct contact with the substrate-receiving surface.

FIELD OF THE INVENTION

The present invention relates to semiconductor manufacturing processes,and in particular relates to an improved susceptor design for epitaxialgrowth on silicon carbide substrates.

BACKGROUND OF THE INVENTION

The present invention relates to the production of epitaxial layers ofsemiconductor materials on silicon carbide substrates. Silicon carbideoffers a number of advantageous physical and electronic characteristicsfor semiconductor performance and devices. These include a wide bandgap,high thermal conductivity, high saturated electron drift velocity, highelectron mobility, superior mechanical strength, and radiation hardness.

As is the case with other semiconductor materials such as silicon, oneof the basic steps in the manufacture of a number of silicon-carbidebased devices includes the growth of thin single crystal layers ofsemiconductor material on silicon carbide substrates. The technique isreferred to as “epitaxy,” a term that describes crystal growth bychemical reaction used to form, on the surface of another crystal, thinlayers of semiconductor materials with defined lattice structures. Inmany cases, the lattice structure of the epitaxial layers (or“epilayers”) are either identical, similar, or otherwise related to thelattice structure of the substrate. Thus, epitaxial growth of eithersilicon carbide epitaxial layers on silicon carbide substrates or ofother semiconductor materials on silicon carbide substrates, is afundamental technique for manufacturing devices based on siliconcarbide.

Silicon carbide is, however, a difficult material to work with becauseit can crystallize in over 150 polytypes, some of which are separatedfrom one another by very small thermodynamic differences. Furthermore,because of silicon carbide's high melting point (over 2700° C.), manyprocesses for working silicon carbide, including epitaxial filmdeposition, often need to be carried out at much higher temperature thananalogous reactions in other semiconductor materials.

Some basic reviews of semiconductor manufacturing technology can befound for example in Sze, Physics of Semiconductor Devices, 2d Ed.(1981), Section 2.2, pages 64-73; or in Dorf, The Electrical EngineeringHandbook, CRC Press, (1993) at Chapter 21 “Semiconductor Manufacturing,”pages 475-489; and particularly in Sherman, Chemical Vapor Depositionfor Microelectronics: Principles, Technologies and Applications, (1987),ISBN O-8155-1136-1. The techniques and apparatus discussed herein can becategorized as chemical vapor deposition (CVD) or vapor phase epitaxy(VPE) in which reactant gases are exposed to an energy source (e.g.heat, plasma, light) to stimulate a chemical reaction, the product ofwhich grows on the substrate.

There are several basic techniques for CVD epitaxial growth, the twomost common of which are the hot (heated) wall reactor and cold wallreactor processes. A hot wall system is somewhat analogous to aconventional oven in that the substrate, the epitaxial growth precursormaterials, and the surrounding container are all raised to the reactiontemperature. The technique offers certain advantages and disadvantages.

The second common conventional technique is the use of a “cold wall”reactor. In such systems, the substrate to be used for epitaxial growthis placed on a platform within a container (typically formed of quartzor stainless steel). In many systems, the substrate is disk-shaped andreferred to as a “wafer.” The substrate platform is made of a materialthat will absorb, and thermally respond to, electromagnetic radiations.

As is known to those familiar with such devices and techniques, thesusceptor's response to electromagnetic radiation is an inductiveprocess in which alternating frequency electromagnetic radiation appliedto the susceptor generates an induced (inductive) current in thesusceptor. The susceptor converts some of the energy from this inductivecurrent into heat. In many systems, the electromagnetic radiation isselected in the radio frequency (RF) range because materials such asglass and quartz are transparent to such frequencies and are unaffectedby them. Thus, the electromagnetic radiation passes through thecontainer and is absorbed by the susceptor which responds by becomingheated, along with the wafer, to the temperatures required to carry outthe epitaxial growth. Because the container walls are unaffected by theelectromagnetic energy, they remain “cold” (at least in comparison tothe susceptor and the substrate), thus encouraging the chemical reactionto take place on the substrate.

A thorough discussion of the growth of silicon carbide epitaxial layerson silicon carbide substrates is set forth for example in U.S. Pat. No.4,912,063 to Davis et al. and U.S. Pat. No. 4,912,064 to Kong et al.,the contents of both of which are incorporated entirely herein byreference.

The use of a cold wall reactor to carry out epitaxial growth, althoughsatisfactory in many respects, raises other problems. In particular,because a semiconductor wafer rests on a susceptor, the wafer side incontact with the susceptor will become warmer than the remainder of thesubstrate. This causes a thermal gradient in the axial direction throughthe wafer. In turn, the difference in thermal expansion within the wafercaused by the axial gradient tends to cause the peripheral edges(typically the circumference because most wafers are disc-shaped) tocurl away from, and lose contact with, the susceptor. As the edges losecontact with the susceptor, their temperature becomes lower than themore central portions of the wafer, thus producing a radial temperaturegradient in the substrate wafer in addition to the axial one.

These temperature gradients, and the resulting physical effects, havecorresponding negative affects on the characteristics of the substrateand the epitaxial layers upon it. For example, if the edges are placedin extreme tension, they have been observed to crack and failcatastrophically. Even if catastrophic failure is avoided, the epitaxiallayers tend to contain defects. At silicon carbide CVD growthtemperatures (e.g. 1300°-1800° C.), and using larger wafers (i.e. twoinches or larger), wafer bending becomes a significant problem. Forexample, FIG. 3 herein plots the values of wafer deflection (H) atvarious axial temperature gradients as a function of the waferdiameters.

Furthermore, because wafers have a finite thickness, the heat applied bythe susceptor tends to generate another temperature gradient along thecentral axis of the wafer. Such axial gradients can both create andexacerbate the problems listed above.

Yet another temperature gradient typically exists between the rearsurface of the substrate wafer and the front surface of the susceptor;i.e. a surface-to-surface gradient. It will thus be understood that bothradiant and conductive heat transfer typically take place betweensusceptors and substrate wafers. Because many susceptors are formed ofgraphite coated with silicon carbide, the thermodynamic driving forcecreated by the large temperature gradients between the susceptor and thesilicon carbide wafers also causes the silicon carbide coating toundesirably sublime from the susceptor to the wafer.

Additionally, because such sublimation tends to promote pin holeformation in the susceptor coating, it can permit contaminants from thegraphite to escape and unintentionally dope the substrates or theepilayers. This in turn ultimately leads to non-uniform doping levels inthe semiconductor material, and reduces the lifetime of the susceptor.The problems created by susceptors which undesirably emit dopants is setforth for example in the background portion of U.S. Pat. No. 5,119,540to Kong et al.

Nevertheless, a need still exists for susceptors that can operate at thehigh temperatures required for silicon carbide processing whileminimizing or eliminating these radial, axial and surface to surfacetemperature gradients, and the associated physical changes and problems.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide asusceptor for minimizing or eliminating radial, axial andsurface-to-surface thermal gradients across a substrate wafer.

The invention meets this object with a susceptor that comprises a firstportion that includes a surface for receiving a semiconductor substratewafer thereon, and a second portion facing the substrate receivingsurface and spaced from the substrate receiving surface with the spacingbeing sufficiently large to permit the flow of gases therebetween forepitaxial growth on a substrate. The spacing remains small enough,however, for the second susceptor portion to heat the exposed face of asubstrate to substantially the same temperature as the first susceptorportion heats the face of the substrate that is in direct contact withthe substrate receiving surface.

In another aspect, the invention is a method for minimizing oreliminating thermal gradients in and around a substrate during epitaxialgrowth by heating a portion of a susceptor that faces, but avoidscontact with, a semiconductor substrate, and that is spaced sufficientlyfar from the substrate to permit the flow of gases between the substrateand the susceptor portion to encourage epitaxial growth on the substratefacing the susceptor portion wherein the susceptor is thermallyresponsive to the irradiating radiation.

The foregoing and other objects, advantages and features of theinvention, and the manner in which the same are accomplished, will bemore readily apparent upon consideration of the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings, which illustrate preferred and exemplary embodiments, andwherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a platform type chemical vapordeposition (CVD) system;

FIG. 2 is a cross-sectional view of a barrel-type CVD system;

FIG. 3 is a graph illustrating the relationship between wafer deflectionand wafer diameter at various temperature gradients;

FIG. 4 is a schematic view of a barrel-type susceptor;

FIG. 5 is a schematic view of wafer deflection and temperaturegradients;

FIG. 6 is a cross-sectional view of one embodiment of a susceptoraccording to the present invention;

FIG. 7 is a partial cross-sectional view of a second embodiment of thesusceptor of the present invention;

FIG. 8 is a cross-sectional view of a pancake-type susceptor;

FIG. 9 is a top plan view of a pancake-type susceptor; and

FIG. 10 is a cross-sectional view of a pancake-type susceptor accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a susceptor for minimizing or eliminatingthermal gradients, including radial, axial, and surface-to-surfacegradients, that affect a substrate wafer during epitaxial growth.Substrates according to the present invention are particularly usefulfor chemical vapor deposition systems as illustrated in FIGS. 1 and 2.FIG. 1 shows a platform or pancake type CVD system broadly designated at20. The system comprises a reactor vessel 21 formed of a material,typically a quartz tube or bell jar, that is substantially transparentto the appropriate frequencies of electromagnetic radiation. A gassupply system is in fluid communication with the reaction vessel 21 andin FIG. 1 is illustrated as the gas injector 22.

The system includes a source of electromagnetic radiation that in FIG. 1is illustrated as the induction coils 25. The operation of suchgenerators and induction coils is generally well known to those ofordinary skill in the art, and will not be discussed further herein indetail. As is also understood in this art, alternative heatingtechniques can include electric resistance heating, radiant lampheating, and similar techniques.

The chemical vapor deposition system shown in FIG. 1 also includes theplatform type susceptor 26 with semiconductor substrates, typicallydisc-shaped wafers 27, thereon. FIG. 1 also illustrates the pumping port30 for evacuating the system as desired.

FIG. 2 illustrates a system that is very similar in terms of its basicoperation, but that is a barrel-type susceptor, rather than apancake-type. In FIG. 2, the CVD system is broadly designated at 32 andshows a reaction vessel 33 which is surrounded by a water jacket 34which circulates water against the walls of the reaction vessel 33. TheCVD system 32 also includes a gas inlet 35 and a gas exhaust 36, a waterinlet 37 and the water outlet 40, and a lifting and rotation assembly 41for the susceptor.

The susceptor itself is broadly designated at 42 and is in the generalshape of a cylinder, although with a shallow slope that gives itsomewhat frustoconical shape. The cylinder is formed of a plurality ofadjacent straight sidewall sections 43 that define the cylinder. Aplurality of wafer pockets 44 are positioned on the sidewalls 43 andhold the semiconductor substrates thereon. The slight incline of thesusceptor walls help keep the wafers in the pockets 44, and improve theuniformity of the resulting epilayers by encouraging more favorable gasflow. FIG. 2 also illustrates the power supply 45 for the induction coilbroadly designated at 46.

FIG. 3 is a graph that helps illustrate the problem addressed by thepresent invention. In FIG. 3, the deflection of a wafer expressed inthousandths of an inch is plotted against the wafer diameter in inchesfor three different temperature gradients (“Delta T”). As noted in FIG.3, the susceptor surface temperature is 1530° C. and the wafer thicknessis 12 mils (0.012 inch). As FIG. 3 illustrates, wafer deflectionrepresents a minimal problem when the diameter of the substrate wafer isabout an inch or less. For larger wafers, particularly those of two,three or even four inches, the deflection becomes more severe, even atrelatively low temperature gradients.

FIG. 4 illustrates a barrel type susceptor similar to that used in theillustration of FIG. 2. Using the same numbering system as FIG. 2, thesusceptor is broadly designated at 42, is made of a plurality ofstraight sidewalls 43 that together define the generally cylindricalshape. The sidewalls 43 include a plurality of wafer pockets 44 forholding the substrate wafer.

FIG. 5 is a schematic illustration of the effects of the temperaturegradients plotted in FIG. 3, and includes the designation of the axialtemperature gradient (ΔT₁) and of the radial gradient (ΔT₂).

FIG. 6 illustrates a susceptor according to the present invention thatis most appropriately used in the barrel type systems illustrated inFIG. 2. In the embodiment illustrated in FIG. 6, the susceptor isbroadly designated at 50 and is a cylinder formed of a plurality ofadjacent straight sidewall sections 51. FIG. 6 illustrates two of thesidewalls in cross-section and one in side elevation. The straightsidewall sections 51, of which there are most typically four, six, oreight, are formed of a material that is thermally responsive to selectedfrequencies of electromagnetic radiation. As noted above, the mostcommon electromagnetic radiation is in the radio frequency range, so thesusceptor material is generally selected to be thermally responsive tosuch RF frequencies. In preferred embodiments, the susceptor 50 isformed of graphite coated with silicon carbide.

In a presently preferred embodiment, the electromagnetic radiation isapplied in the 8-10 kilocycle range using a solid state power supplythat takes advantage of the inherent efficiencies of solid statetechnology. Those familiar with inductive CVD processes will alsorecognize that thicker susceptor walls require lower frequencies toachieve the most efficient penetration.

In the embodiment illustrated in FIG. 6, the susceptor 50 includes aplurality of wafer pockets 52 on the inner circumference of thecylinder. Thus, when the susceptor 50 is heated, the facing wallsradiantly heat the front of the wafers while the susceptor heats therear. In other words, the facing walls directly (i.e., actively ornon-passively) heat one another in response to exposure toelectromagnetic radiation. As FIG. 6 illustrates, in this embodiment,the sidewalls 51 preferably define an inverted truncated cone with arelatively shallow slope as compared to a true cylinder. As notedearlier, the shallow slope in the sidewalls 51 makes it somewhat easierto retain the wafers in the pockets 52 during chemical vapor deposition,and also helps provide a proper flow pattern for the CVD gases.

FIG. 7 illustrates a next embodiment of the invention in which thesusceptor comprises a first cylinder (or “barrel”) broadly designated at54. The cylinder is defined by a plurality of adjacent straight sidewallsections 55, and is formed of a material that is thermally responsive toselected frequencies of electromagnetic radiation. The cylinder 54includes a plurality of wafer pockets 56 on the outer surface of thesidewall sections 55.

A second cylinder broadly designated at 57 surrounds the first cylinder54 and defines an annular space A between the first and secondcylinders. The second cylinder 57 is likewise made of a material that isthermally responsive to the selected frequencies of electromagneticradiation, and the annular space between the first and second cylinders(54, 57) is sufficiently large to permit the flow of gases therebetweenfor epitaxial growth on substrates in the wafer pockets 56, while smallenough for the second cylinder 57 to heat the exposed face of substratesto substantially the same temperature as the first cylinder 54 heats thefaces of substrates that are in direct contact with the first cylinder(i.e., the second cylinder directly or actively heats the substrate andfirst cylinder in response to electromagnetic radiation).

The first and second cylinders 54, 57 can be formed of either the sameor different materials. If used in a barrel type susceptor system asillustrated in FIG. 2, the second cylinder 57 tends to heat the firstcylinder 54 to encourage the cylinders to reach substantially the sametemperatures. As in other embodiments, each of the cylinders is mostpreferably formed of graphite coated with silicon carbide.

It will be understood that the use of a silicon carbide coating on suchsusceptors is a function of the ceramic properties of polycrystallinesilicon carbide and is otherwise not related to its semiconductorproperties. Thus, susceptors made of stainless steel, graphite, graphitecoated with silicon carbide, or silicon carbide, are typically used inthe semiconductor industry for CVD processes.

FIGS. 8, 9 and 10 illustrate another susceptor according to the presentinvention. FIGS. 8 and 9 illustrate, in cross-section and top plan viewrespectively, a pancake or plate-shaped susceptor broadly designated at60. The susceptor 60 has a top surface 61 for receiving semiconductorsubstrate wafers thereon. In this embodiment, the invention furthercomprises a horizontally disposed second susceptor portion 63 parallelto and above the wafer receiving surface 61 of the first susceptorportion 60. Both of the susceptor portions 60 and 63 are formed ofmaterials that are thermally responsive to selected frequencies ofelectromagnetic radiation, and as in the previous embodiments, arepreferably formed of the same material to be responsive to the samefrequencies of electromagnetic radiation. Most preferably, bothsusceptor portions 60 and 63 are formed of graphite coated with siliconcarbide. As in the previous embodiments, the spacing designated B (FIG.10) between the two portions 60, 63 is sufficiently large to permit theflow of gases therebetween for epitaxial growth on a substrate on thesurface 61, while small enough for the second susceptor portion 63 toheat the exposed face of a substrate to substantially the sametemperature as the first susceptor portion 60 heats the face ofsubstrate that is in direct contact with the substrate receiving surface61. As illustrated in FIGS. 8, 9 and 10, the top surface 61 of the firsthorizontal susceptor portion 60 preferably includes a plurality of waferpockets 64.

In each of these embodiments, it will be understood that the twosusceptor portions can be connected to one another, or separate portionsof a single susceptor, or independent pieces as may be desired ornecessary under various circumstances. Additionally, the optimum spacingbetween the substrate portions can be determined by computer modeling oractual practice, and without requiring undue experimentation.

In another aspect, the invention comprises a method for minimizing oreliminating thermal gradients in a substrate during epitaxial growth. Inthis aspect, the invention comprises irradiating a susceptor, or asusceptor portion, that faces, but avoids contact with, a semiconductorsubstrate wafer, and that is spaced sufficiently far from the wafer topermit the flow of gases between the substrate and the facing susceptorto thereby encourage epitaxial growth on the substrate facing thesusceptor portion. As in the structural embodiments, the susceptor isthermally responsive to the irradiating radiation.

As further set forth with respect to the structural aspects of theinvention, the invention also preferably comprises concurrentlyirradiating a separate susceptor portion upon which the wafer rests sothat the exposed face of the substrate is heated to substantially thesame temperature as is the face of the substrate that is in directcontact with the other susceptor portion.

The method further comprises the steps of directing source gases thatflow between the heated susceptor portions. If the epitaxial layers areto be formed of silicon carbide, the method preferably comprisesdirecting silicon and carbon containing source gases such as silane,ethylene, and propane.

Where other materials, such as Group III nitrides, are to form theepitaxial layers on the silicon carbide, the step of directing sourcegases can include directing gases such as trimethyl aluminum, trimethylgallium, trimethyl indium, and ammonia.

In preferred embodiments, the method also comprises the step ofpreparing the substrate surface for growth. As set forth in more detailin the references incorporated above, such preparation can comprisesteps such as oxidizing the surface followed by a chemical etching stepto remove the oxidized portion leaving a prepared surface behind, oralternatively, dry etching the silicon carbide surface to prepare it forfurther growth. As in most epitaxial growth technique, surfacepreparation further typically comprises lapping and polishing thesubstrate surface prior to the oxidation or etching steps.

In the drawings and specifications, there have been disclosed typicallypreferred embodiments of the invention and, although specific terms havebeen employed, they have been used in a generic sense and in descriptivesense only, and not for purposes of limitation, the scope of theinvention being set forth in the following claims:

That which is claimed:
 1. A susceptor for minimizing or eliminatingthermal gradients across a substrate wafer, said susceptor comprising: afirst cylinder formed of a plurality of adjacent straight sidewallsections that define the cylinder, said cylinder being formed of amaterial that is thermally responsive to selected frequencies ofelectromagnetic radiation; a plurality of wafer pockets on the outersurface of said sidewall sections; and a second cylinder surroundingsaid first cylinder and defining an annular space between said first andsecond cylinders, said second cylinder being made of a material that isthermally responsive to selected frequencies of electromagneticradiation, with the annular space between said first and secondcylinders being sufficiently large to permit the flow of gasestherebetween for epitaxial growth on substrates in said wafer pockets,while small enough for said second cylinder to heat the exposed face ofsubstrates to substantially the same temperature as said first cylinderheats the faces of substrates that are in direct contact with said firstcylinder wherein said first and second cylinders are formed of the samematerial and are responsive to the same frequencies of electromagneticradiation.
 2. A susceptor according to claim 1 wherein said first andsecond cylinders are thermally responsive to radio frequencyelectromagnetic radiation.
 3. A susceptor according to claim 1 whereinsaid first cylinder is formed of graphite coated with silicon carbide.4. A susceptor according to claim 1 wherein said second cylinder isformed of graphite coated with silicon carbide.